In 3G and Long Term Evolution (LTE), a great deal of care is taken in planning scrambling codes for 3G and for planning Scrambling Codes (SC) and Physical Cell Identities (PCI) for cellular networks. SCs and PCIs are non-unique identifier codes for cells in a telecommunications network. There are 512 primary SCs available for 3G and 504 PCIs available for LTE. The limited number PCIs and scrambling codes are reused in a network, and network planners attempt to maximize the distance between cells that use the same PCIs.
When cells with the same PCI are close to one another, network entities that rely on the PCIs to identify cells may confuse the cells with one another. Such an event is referred to here as a scrambling code or PCI conflict. Conflicts can result in degraded performance that manifests through handover failures and ghosting, which is a form of false detection. Both of these problems lead to elevated dropped call rates.
In most networks around the world, groups of codes are allocated so that physical distance is the primary value used to distribute PCIs in a reuse scheme. When engineers typically implement reuse of PCIs, codes are not re-allocated until some distance away.
Conventional approaches often fail in several respects. First, with coastal markets, those with large bodies of water, and markets with highly varied terrain, distance is not as effective for minimizing identifier conflicts. For example, when two cells are separated by a substantial body of water, signals from those cells may experience a high level of conflicts despite being relatively far apart from one another.
Second, in relatively dense markets with urban canyons and vertical layering, distances used for lower density urban and suburban settings can lead to anomalous results. Variations in cell density compromise the efficacy of distance-based reuse planning.
Third, with markets that are adding many frequency layers, it becomes difficult to find available identifiers and maintain an effective plan. There are often tens of thousands of cells in a network. This is difficult to manage, and mistakes occur. Many ad-hoc changes happen in a network, which further complicates matters.